1. Field of the Invention
The present invention relates to a sensorless spindle motor control apparatus used in, e.g., a magnetic disk apparatus.
2. Description of the Related Art
A compact magnetic disk apparatus adopts a sensorless spindle motor as a motor for rotating a recording medium. The sensorless spindle motor has no sensor for detecting the rotor position. Therefore, in the magnetic disk apparatus using the sensorless spindle motor, the rotor position is detected on the basis of a counter electromotive voltage generated in each coil.
No counter electromotive voltage is generated unless the rotor is rotated. For this reason, when the spindle motor is started, the phases are sequentially to rotate the rotor, and the initially excited phase is not dependent on the rotational position of the rotor to rotate the rotor. This rotation is called a forced rotation.
In the forced rotation mode, if the length of time for sequentially exciting the phases is proper, the rotor is rotated. However, if the time for sequentially exciting the phases is too short, an out-of-phase state occurs, and the rotor is not rotated. Conversely, even though the length of time for sequentially exciting the phases is proper, if the load on the rotor is increased for any cause, the the length of time for sequentially exciting the phases becomes improper, and the rotor is not rotated. In this manner, when the length of time for sequentially exciting the phases is not proper, the out-of-phase state cannot be removed even after the forced rotation is retried any number of times, and the motor cannot be started.
Upon starting of the spindle motor, when the phases of the coils are excited in turn, the rotor may be attracted in a direction opposite to a normal rotational direction depending on the initial rotor position, and may be kept rotated in the reverse direction. In general, upon execution of the forced rotation, one of the phases of the coils is excited to align the rotor at an arbitrary position. This operation is called alignment excitation.
FIG. 10 shows the relationship between the rotor position and the torque upon starting of a motor. The ordinate corresponds to a torque axis, and a relative value is plotted when the maximum value of a torque to be generated is assumed to be "1". The abscissa corresponds to an angle axis, and an electrical degree between an excitation phase and the rotor when one period of a change in phase is assumed to be 360.degree. is plotted. A torque characteristics curve a represents the relationship between the relative positions between the excitation phase and the rotor and the generated torque.
As can be apparent from FIG. 10, when the electrical degree falls within a range of 0.degree..+-.30.degree. and a range of 180.degree..+-.30.degree., a torque only 1/2 the peak value is generated. Normally, the motor can be started even by the 1/2 torque without posing any problem. However, when the static friction of the rotor, e.g., the static friction between a magnetic disk and a magnetic head, is considerably increased due to, e.g., aging of the apparatus, the rotor cannot be rotated. More specifically, even when the forced rotation is retried any number of times, if the phase to be excited is always the same, a sufficient torque cannot be obtained, and the motor cannot be started.